title: Glycosylation of closely spaced acceptor sites in human glycoproteins.
abstract: Asparagine-linked glycosylation of proteins by the oligosaccharyltransferase (OST) occurs when acceptor sites or sequons (N-x≠P-T/S) on nascent polypeptides enter the lumen of the rough endoplasmic reticulum. Metazoan organisms assemble two isoforms of the OST that have different catalytic subunits (STT3A or STT3B) and partially non-overlapping cellular roles. Potential glycosylation sites move past the STT3A complex, which is associated with the translocation channel, at the protein synthesis elongation rate. Here, we investigated whether close spacing between acceptor sites in a nascent protein promotes site skipping by the STT3A complex. Biosynthetic analysis of four human glycoproteins revealed that closely spaced sites are efficiently glycosylated by an STT3B-independent process unless the sequons contain non-optimal sequence features, including extreme close spacing between sequons (e.g. NxTNxT) or the presence of paired NxS sequons (e.g. NxSANxS). Many, but not all, glycosylation sites that are skipped by the STT3A complex can be glycosylated by the STT3B complex. Analysis of a murine glycoprotein database revealed that closely spaced sequons are surprisingly common, and are enriched for paired NxT sites when the gap between sequons is less than three residues.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
authors.forename: Shiteshu
authors.initials: S
authors.lastname: Shrimal
authors.forename: Reid
authors.initials: R
authors.lastname: Gilmore
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015870:Gene Expression; D006023:Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006031:Glycosylation; D006242:Haptoglobins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006466:Hemopexin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006602:Hexosyltransferases / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational
chemical_list: D006023:Glycoproteins; C485359:HP protein, human; D006242:Haptoglobins; D008565:Membrane Proteins; D006466:Hemopexin; D006602:Hexosyltransferases; C099573:STT3A protein, human; C551978:STT3B protein, human
keywords: Asparagine-linked glycosylation; Endoplasmic reticulum; Oligosaccharyltransferase